The classic physiological role for 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) is to maintain appropriate calcium homeostasis by ensuring adequate intestinal absorption of dietary calcium. However, 1,25(OH)2D3 and its cognate receptor, the vitamin D receptor (VDR), also have critical roles in the skin and in keratinocyte biology. In fact, VDR is a clinically relevant target for the use of 1,25(OH)2D3 analogs in the treatment of psoriasis, a hyperproliferative disorder of the skin. The focus of this research proposal is on the vitamin D endocrine system in epidermal keratinocytes and, specifically, on the use of murine models of the vitamin D endocrine system as tools to explore the in vivo role and identify potential mechanisms of vitamin D action in the skin. In particular, the VDR knockout mouse (VDRKO) is highly susceptible to chemically-induced skin tumorigenesis. In contrast, our preliminary data show that genetic ablation of CYP27B1, the gene encoding the renal 1alpha- hydroxylase (1aOHase), the enzyme that generates the bioactive 1,25(OH)2D3 hormonal ligand, is completely resistant to chemically-induced skin tumorigenesis. These data as well as published data on hair follicle cycling in these mouse models and in humans, point to a novel ligand-independent role for VDR in keratinocytes. The molecular details involved in the tumor suppressor role for unliganded VDR in the skin are completely unknown. Our preliminary in vivo and in vitro data support a model in which the VDR-RXR heterodimer is activated by EGF in the absence of its 1,25(OH)2D3 ligand selectively in keratinocytes. EGF treatment drives heterodimerization of VDR and RXR in the absence of the 1,25(OH)2D3 ligand and this in turn promotes heterodimer binding to select keratinocyte genes involved in diverse processes such as hair follicle cycling and in protecting skin tissue from tumorigenesis initiated by DNA-damaging agents. Importantly, this mechanism exists for select target genes, i.e., not all established VDR target genes are regulated by unliganded VDR. It is this class of undefined genes whose expression is controlled by unliganded VDR-RXR that need to be defined in order to understand the in vivo biologies of the VDRKO and 11OHaseKO mice. In order to define the genes that are direct targets for unliganded VDR-RXR in keratinocytes, we will use gene expression array analysis of mouse keratinocytes and purified bulge stem cells from WT, VDRKO, and 1aOHaseKO mice. PUBLIC HEALTH RELEVANCE: The molecular details involved in the tumor suppressor role for unliganded VDR in the skin are completely unknown. Our working hypothesis states that EGF signaling in keratinocytes impinges on VDR to promote VDR-RXR heterodimerization and transactivation of select target genes in the absence of the 1,25(OH)2D3 ligand. The main goal of this proposal is to define target genes for unliganded VDR-RXR heterodimers in keratinocytes.